How come when people are in court for murder and jury are deciding guilty or not - why can't lie detectors be used?

I think because a lot of people can fool a lie detector. Lot's of people can control their emotions to fool a lie detector.

First of all lie detectors are a joke as I've been hooked up to them.  Second everyone lies the cops will lie to get a confession out of u

As Rooster and Aux said ...

Plus, you can't be compelled to testify against yourself - and the courts assume you are innocent until proven guilty - - and a lie detector would violate both those rights.  Even for a witness.

I don't suppose I for one would consider that evidence in a court decision. "Man sent to jail for murder; lie detector used for conviction."

Lie detectors are not as accurate as we think it is. Since there are methods on how you can deceive the lie detector leading to defective results.

Lie detectors do not measure the truth; this machine measures your physical response to questions.  So, if you are being questioned about a murder or robbery or another criminal event, it will register how upset you are.  Whether or not you are guilty, being in the system and being identified as a criminal with a background will be upsetting and will register on the machine.

A lie detector does not measure guilt.  It measures response, not intent, motivation, or access.